Foreword

Multidrug-resistant organisms (MDROs) are a serious public health concern, as described in CDC’s 2013 and 2019 AR Threats Reports. Infections with MDROs cause an estimated 2.8 million infections and more than 35,000 deaths per year (source).

CDC’s Antimicrobial Resistance (AR) Lab Network was established in 2016 to support testing for Targeted MDROs, which are resistant to most or all available antimicrobials and have the potential to spread widely. Current Targeted MDROs that the AR Lab Network provides support for testing are

  • Candida auris
  • Carbapenemase-producing organisms (CPOs), including:
    • Acinetobacter baumannii
    • Pseudomonas aeruginosa
    • Bacteria from the Enterobacterales order
  • Pan-resistant organisms

Expanding testing for antimicrobial resistance with CDC’s support has allowed public health to detect and respond to MDROs in a timely manner, to limit spread of resistant organisms.

This report is a compilation of testing performed in 2024, with some historical comparative data from 2020—2024. The Midwest Region AR Laboratory is based in Madison, Wisconsin, at the Wisconsin State Laboratory of Hygiene (WSLH). It serves the six states of the Midwest Region: Illinois, Indiana, Kentucky, Michigan, Ohio, and Wisconsin.

This report includes data on colonization testing for CPOs and Candida auris, as well as isolate testing for Candida species. WSLH also performs testing on bacterial isolates of CPOs, but those data have not been included, due to Wisconsin isolates being over-represented. For more information on MDROs in Wisconsin, please see the Wisconsin Department of Health Services’ page on Reportable MDROs.

Colonization Testing Overview

A person who is colonized with an MDRO is carrying the organism in or on their body, without it causing clinical illness. People who are colonized with MDROs can be colonized for long periods of time, even indefinitely. Many people who are colonized also later have infections with those MDROs, which can cause serious illnesses.

Both colonized patients and those with active infections can spread the MDROs to other patients in the healthcare environment. While clinical illness is usually detected in the course of diagnosis and treatment of the infection, many colonized patients go undetected. Detection of colonization is an important tool to prevent the spread of MDROs.

WSLH supports colonization testing in the Midwest Region. Currently, WSLH performs colonization testing for carbapenemase-producing organisms from carbapenem-resistant Enterobacterales (CRE), carbapenem-resistant Pseudomonas aeruginosa (CRPA), and carbapenem-resistant Acinetobacter baumannii (CRAB). WSLH also performs colonization testing for Candida auris. Testing has increased over time, especially following the COVID-19 pandemic.

Midwest Region all colonization testing, 2020—2024

CRE/CRPA colonization testing

Carbapenemases are enzymes produced by some bacteria that make them resistant to important antibiotics. Some common carbapenemases (IMP, KPC, NDM, OXA-48, and VIM) are most frequently found in bacteria that can colonize the gastrointestinal tract, including species from the order Enterobacterales such as E. coli, Klebsiella pneumoniae, and Enterobacter cloacae. These carbapenemases can also be found in Pseudomonas aeruginosa.

WSLH uses CARBA-R, a sample-to-answer real-time PCR test, to detect these common carbapenemases directly from rectal swabs. In 2024, 1807 (11.0%) of the 16,371 specimens tested using this assay were positive for at least one carbapenemase.

Submitters of specimens for CRE/CRPA colonization testing, 2024

CRE/CRPA colonization testing by state, 2020—2024

CRE/CRPA colonization test results, 2020—2024

Genes detected in CRE/CRPA colonization screening, 2020—2024

This table shows the breakdown of genes when multiple genes were detected from a single specimen, 2020—2024.

Genes detected

KPC / NDM

116

KPC / VIM

43

KPC / OXA-48

13

NDM / OXA-48

8

KPC / NDM / VIM

6

NDM / VIM

4

KPC / NDM / OXA-48

1

KPC / OXA-48 / VIM

1

CRAB colonization testing

Carbapenemase-producing carbapenem-resistant Acinetobacter baumannii (CP-CRAB) often contain carbapenemases that are unique to Acinetobacter species. WSLH has PCR tests for these CRAB-specific carbapenemases, which include OXA-23-like, OXA-24/40-like, and OXA-58-like genes. WSLH also tests CRAB for other carbapenemases, including NDM, KPC, IMP, VIM, and OXA-48-like genes. Colonization testing for CP-CRAB in 2024 was culture-based. WSLH performs CRAB testing primarily from axilla/groin swabs, but is also able to perform testing on rectal and tracheostomy swabs.

Submitters of specimens for CRAB colonization testing, 2024

CRAB colonization testing by state, 2020—2024

CRAB colonization test results, 2020—2024

Midwest Region carbapenemase genes in CRAB, 2020—2024

Genes detected

OXA-24/40

1,619

OXA-23

673

NDM

178

NDM / OXA-23

49

NDM / OXA-24/40

33

OXA-23 / OXA-24/40

11

NDM / OXA-23 / OXA-24/40

1

Candida auris colonization testing

Candida auris is a fungal pathogen that has emerged as a serious healthcare-associated infection (HAI) since its discovery in 2009. C. auris can colonize patients’ skin and the healthcare environment, leading it to spread undetected in some facilities. Colonization testing for C. auris involves PCR-based testing of axilla/groin swabs.

Submitters of specimens for C. auris colonization testing, 2024

C. auris colonization testing by state, 2020—2024

C. auris colonization test results, 2020—2024

Candida species isolate testing

WSLH performs isolate testing for Candida auris and for some unusual Candida species from the Midwest region. This includes species identification via MALDI-TOF and antifungal susceptibility testing (AFST) using a custom broth microdilution panel for azole and echinocandin antifungals, and an E-test for Amphotericin B.

Midwest Region Candida isolate testing, 2020—2024

Midwest Region Candida isolate testing by state, 2020—2024

C. auris AFST

C. auris micafungin susceptibility

Micafungin is an echinocandin antifungal, a class of drugs that is relatively well-tolerated and important clinically. Echinocandin resistance among Candida auris is rare, but concerning. Resistance is sometimes caused by mutations in the FKS 1 gene. The current breakpoint for resistance to micafungin is 4 µg/mL.

C. auris fluconazole susceptibility

Azole antifungals included in WSLH’s AFST panel include fluconazole, voriconazole, itraconazole, isavuconazole, and posaconazole. CDC only provides MIC interpretations for fluconazole, which has a breakpoint at ≥ 32 µg/mL.

C. auris Amphotericin B susceptibility

Amphotericin B is a powerful antifungal, but one that often causes severe side effects in patients. The most recent breakpoints from CDC consider any isolates with an MIC of ≥2 µg/mL to be resistant.

C. auris WGS

WSLH began performing whole genome sequencing (WGS) on Candida auris isolates in 2024. Currently, WSLH sequences the first clinical isolate received per patient. WSLH also sequences some colonization specimens upon request. The WGS analysis process identifies to which of the six known clades each isolate belongs. The Midwest Region sees isolates from Clades I, III, IV.

WSLH’s process also looks for a mutation in the FKS 1 gene that potentially confers resistance to echniocandins. In isolates tested that were collected in 2024, no FKS 1 mutations were identified.

Appendix A - Acronym guide

  • AR — Antimicrobial resistance
  • CDC — Centers for Disease Control and Prevention
  • CP — Carbapenemase-producing
  • CPO — Carbapenemase-producing organism
  • CRAB — Carbapenem-resistant Acinetobacter baumannii
  • CRE — Carbapenem-resistant Enterobacterales
  • CRPA — Carbapenem-resistant Pseudomonas aeruginosa
  • KPC — Klebsiella pneumoniae carbapenemase
  • IMP — Imipenemase
  • MDRO — Multidrug-resistant organism
  • NDM — New Delhi metallo-beta-lactamase
  • OXA — Oxacillinase
  • VIM — Verona integron-mediated metallo-beta-lactamase
  • WSLH — Wisconsin State Laboratory of Hygiene